Inserted gallery preview disables TinyMCE content editing

Description

Using the new media UI -- if I insert a gallery of images into a post without any existing content in the editor, it's not possible to add more text around the preview once it's inserted. The same is true of adding more text after a a gallery preview, even if there is text above it.

To reproduce:

Start a new post, don't add any content into the TinyMCE editor. Using the new media UI, add some images and create-insert a new gallery. Try adding text before or after the gallery preview in the editor.